Vol. XXIXI 

 1912 



] McAtee, Contents of Bird Stotnachs. 459 



certain average bulk of food. We thus have for each species a 

 standard of food consumption, by which we can computate its 

 demands for any multiple or fraction of the standard period. The 

 number of insects, seeds, etc., consumed cannot be so standardized. 

 This is true partly because, many of the items cannot be counted 

 fas explained above), and partly because the elements of food vary 

 so much in size. Thus a bird may take as a certain proportion of 

 its subsistence, 10 grasshoppers, or it may take instead 50 beetles, 

 or 1000 ants. In view of this fact what does it mean to say so 

 many birds took a certain number of beneficial and a certain num- 

 ber of injurious insects. Can we possibly learn by numbers their 

 relation to the whole food? It is perfectly evident that a bird 

 requires, not a certain number of insects and seeds per day, but a 

 certain average bulk of food, which may be made up of an exceed- 

 ingly wide variety of items of very diverse sizes. It follows there- 

 fore that we can estimate the importance of any element of the diet, 

 only when we know its proportion to the standard requirement. 



We must express ourselves in terms of bulk also when we desire 

 to state the amount of damage done to crops. The cultivator 

 wishes to know how many quarts of cherries or pecks of grain the 

 birds are apt to destroy in a year. We can make these estimates 

 with greatest accuracy when we know the proportion of the annual 

 food of birds, composed of these items. 



Suppose, using the numerical system, we say we have examined 

 100 Crow stomachs and found in them 675 kernels of corn. What 

 does this mean? Can we learn by a numerical comparison with 

 the grasshoppers, or acorns eaten, what proportion of the yearly 

 food consists of corn? The case is different if we can say that corn 

 constituted 15 per cent of the food of these 100 Crows. We then 

 know something a,bout the Crows' relative taste for corn, know that 

 they could have taken much more, but chose to eat other things. 

 The farmer in the locality in which they were collected knows from 

 such a statement about what damage he may expect from Crows. 



Without the percentage-by-bulk system the writer would have 

 been unable to make the following statements ^ regarding the food 

 of the Black-headed Grosbeak, namely : " that the animal food of 



1 Bull. 32, Biol. Siirvey, 1908, p. 76. 



